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Component Model Strawman: Element Registration
Overview
With the Element Registration mechanism. page authors can tell new element names and its behavior to agents.
The element element and HTMLRegistrationElement
The HTMLRegistrationElement represents an author-registered element definition.
Here is the simplest example which registeres an element named "x-comment" whose instance implements HTMLElement
// Imperative API
var element = document.createElement("element");
element.name = "x-comment";
document.head.appendChild(element);
// Shorter form.
document.elements.register("x-comment");
<!-- Markup API -->
<head>
<element name="x-comment"></element>
</head>
Scripting
The script element
The element element allows script elements in its children. These script blocks are executed over a global object. Before the evaluation, HTMLRegistrar.registering is set to the enclosing element element. These script blocks can be used for providing lifecycle callback definitions for the registering element.
<head>
<element name="x-comment">
<script>
var shouldBeXComment = document.elements.registering.name;
<script>
</element>
</head>
The creator callback
The HTMLRegistrationElement provide a pair of lifecycle callbacks for the element instance creation including the tree construction.
The first callback, creator is invoked when an agent need a new element instance of the registered element. creator callback should return new element instance which matches the registered element specification.
// Imperative API
var element = document.createElement("element");
element.name = "x-comment";
element.creator = function() { return new HTMLElement("x-comment"); };
document.head.appendChild(element);
// Shorter form.
document.elements.register("x-comment", function() { ... });
<!-- Markup API -->
<head>
<element name="x-comment">
<script>
document.elements.registering.creator = function() {
return new HTMLElement("x-comment");
};
</script>
</element>
</head>
If the callback function has HTMLElement in its prototype chain, it is invoked as a constructor. Thus following three markup and one imperative examples have same meaning.
<!-- Markup 1 -->
<element name="x-comment">
<script>
class Comment : HTMLElement {
....
};
document.elements.registering.creator = Comment;
</script>
</element>
<!-- Markup 2 -->
<element name="x-comment">
<script>
class Comment : HTMLElement {
....
};
document.elements.registering.creator = function() { return new Comment(); };
</script>
</element>
// Imperative
class Comment : HTMLElement {
...
};
document.elements.register("x-comment", Comment);
If no creator callback is given, its default behavior is something like this:
<element name="x-comment">
<script>
var element = this;
document.elements.registering.creator = function() {
return new HTMLElement(element.name);
};
</script>
</element>
The creator attribute
A creator callback can also be set by the creator attribute on element element.
Once the attribute value is set, that is invoked as an expression.
The expression should return a function object.
And the returned function object is set to creator property of the element element.
<head>
<element name="x-comment"
creator="function() { new HTMLElement('x-comment'); }">
</element>
</head>
This is especially useful if you have a class-style constructor. It can be given like this.
<element name="x-comment" creator="Comment">
<script>
class Comment : HTMLElement {
....
};
</script>
</element>
The setup callback
The second lifecycle callback, setup is called after an instance creation. If the instance is created by the agent's tree construction, it will be called as a part of the "close" phase. On setup, the attributes and child elements for the element are already set by the agent. So this callback is useful for building its visual like the shadow tree.
// Imperative API
var element = document.createElement("element");
...
element.setup = function() {
this.shadow = new ShadowRoot(this);
...;
};
...
<!-- Markup API -->
<head>
<element name="x-comment">
<script>
...
document.elements.registering.setup = function() {
// "this" points the newly created element instance.
this.shadow = new ShadowRoot(this);
...;
};
</script>
</element>
</head>
If no setup callback is given by the author, agent invokes the default behavior. It is something like this:
function defaultSetup() {
if (this.setup instanceof Function)
this.setup();
}
So if we define setup for the registered element, it will be called as a setup callback.
<element name="x-comment" creator="Comment">
<script>
class Comment : HTMLElement {
setup(shadow) {
// should be called.
}
}
</script>
</element>
Styling
The element element also allows the style element as its children. The style given by the element is a part of document stylesheet. Authors can use the style element to provide the style for the registering element.
<head>
<element name="x-comment">
<style>
x-comment {
color: gray;
}
</style>
</element>
</head>
Shadow Tree Construction and the template element
The element element helps to construct a shadow tree for newly created elements. If element element has a template element as its child, The agent creates shadow tree from the template element before invoking the setup callback, then invokes it with the shadow root.
<!-- Markup API -->
<head>
<element name="x-comment">
<template>
<div>...</div>
</template>
<script>
...
document.elements.registering.setup = function(shadow) {
// setup using the shadow instance.
};
</script>
</element>
</head>
HTMLRegistrationElement interface
[Callback=AcceptConstructor]
interface HTMLRegistrationCreatorCallback {
HTMLElement create();
};
[Callback]
interface HTMLRegistrationSetupCallback {
void setup(ShadowRoot shadow = null);
};
[Constructor]
interface HTMLRegistrationElement {
attribute String name;
attribute HTMLRegistrationCreatorCallback creator;
attribute HTMLRegistrationSetupCallback setup;
HTMLElement create();
};
HTMLRegistrationElement#create() instantiates an registered element instance.
* TODO: The parameters of create() should match to the standard element constructor parameter list.
HTMLRegistrar and document.elements
HTMLRegistrar is an object oriented way to access registered element definitions. It provides a keyed-collection like access to each registered element. It also provides some utility method which helps idiomatic element registration and its use.
The object exists per document, and is accessible through the Document#elements property.
[NoInterfaceObject]
interface HTMLRegistrar {
getter HTMLRegistrationElement (DOMString name);
void register(DOMString name, [Optional] HTMLRegistrationCreatorCallback creator);
readonly attribute HTMLRegistrationElement registering;
};
The getter
The getter returns HTMLRegistrationElement whose name attribute matches name parameter. unless returns null.
The getter can be used like this:
var newCommentElement = document.elements["x-comment"].create();
The register method
The HTMLRegistrar#register method is a shortcut for following:
var element = document.createElement("element");
element.name = name;
element.creator = creator;
document.head.appendChild(element);
The registering attribute
For each script block which is a child of element is executed, The HTMLRegistar#registering is set to the element element.